Tetrahedral M4(μ4-O) Motifs Beyond Zn: Efficient One-Pot Synthesis of Oxido–Amidate Clusters via a Transmetalation/Hydrolysis Approach

While zinc μ4-oxido-centered complexes are widely used as versatile precursors and building units of functional materials, the synthesis of their analogues based on other transition metals is highly underdeveloped. Herein, we present the first efficient systematic approach for the synthesis of homometallic [M4(μ4-O)L6]-type clusters incorporating divalent transition-metal centers, coated by bridging monoanionic organic ligands. As a proof of concept, we prepared a series of charge-neutral metal-oxido benzamidates, [M4(μ4-O) (NHCOPh)6] (M = Fe, Co, Zn), including iron(II) and cobalt(II) clusters not accessible before. The resulting complexes were characterized using elemental analysis, FTIR spectroscopy, magnetic measurements, and single-crystal X-ray diffraction. Detailed structural analysis showed interesting self-assembly of the tetrahedral clusters into 2D honeycomb-like supramolecular layers driven by hydrogen bonds in the proximal secondary coordination sphere. Moreover, we modeled the magnetic properties of new iron (II) and cobalt (II) clusters, which display a general tendency for antiferromagnetic coupling of the μ4-O/μ-benzamidate-bridged metal centers. The developed synthetic procedure is potentially easily extensible to other M(II)-oxido systems, which will likely pave the way to new oxido clusters with interesting optoelectronic and self-assembly properties and, as a result, will allow for the development of new functional materials not achievable before.


■ INTRODUCTION
Molecular metal-oxido clusters have gained attention as versatile building units of a wide variety of functional materials based on both coordination 1,2 and noncovalent supramolecular networks. 3,4 The most prominent subsets of metal-oxido clusters include trinuclear μ 3 -oxido-centered complexes, tetranuclear μ 4 -oxido clusters, and multinuclear polyoxidometalates (POMs) (Figure 1a). Among them, clusters comprising a highly symmetrical tetrahedral [M 4 (μ 4 -O)] 6+ core (M = divalent metal) stabilized by six monoanionic bidentate organic ligands are of particular interest due to their multifaceted chemistry arising from the presence of several spots of structural tailorability, including divalent metal centers in the tetrahedral core, the character of anchoring groups, and the organic backbone of ligands in the secondary coordination sphere (Figure 1b). 5,6 Such diversity of structural features results in tuneable optoelectronic and coordination properties, which turns μ 4 -oxido compounds into suitable candidates for magnetic, 7 6 ]-type complexes can be regarded as discrete soluble intermediates between simple monomers and polymeric lattices of various hybrid organic−inorganic architectures. As such, they are widely used as model systems for studying properties of more complex systems including MOFs 16,17 and hybrid metal oxide nanoparticles, 18,19 as well as effective precursors of functional materials. 5,6,20−26 Nevertheless, the development of materials containing the [M 4 (μ 4 -O)] structural motif beyond zinc clusters is hampered by the lack of facile access to this class of complexes comprising metal centers of desired optoelectronic, catalytic, or magnetic properties. For instance, access to homometallic isoreticular MOF-5 analogues based on metals other than zinc is still very limited. To the best of our knowledge, three Co(II) pyrazolate frameworks are the only examples of such materials obtained via direct synthesis from inorganic salts. 27−29 Strikingly, this approach failed in the case of isoreticular frameworks based on dicarboxylate linkers. We also note that MOF-5(Co) and MOF-5(Be) were obtained from well-defined metal-oxido precursors using the controlled SBU approach, 20 and cobalt (II) MOF-5 analogues were obtained by a post-synthetic transmetalation. 17,30 However, the latter process was not effective for other metal ions.
To date, readily accessible zinc μ 4 -oxido clusters have been the subject of the most extensive investigations. For example, the most common zinc-oxido carboxylates are easily prepared by either thermal decomposition of zinc carboxylates via elimination of the corresponding anhydride 31 or in alkaline solutions of zinc salts with water acting as a source of the O 2− ion. 10,32,33 An increased control over the process of zinc-oxido carboxylates formation was achieved by using homoleptic zinc carbamates as well-defined precursors acting simultaneously as water deprotonation agents (Figure 2a). 34−37 Another approach based on well-defined precursors utilizes diorganozinc species, which are reacted with the appropriate proligand, and then are exposed to dioxygen 38−40 or undergo hydrolysis by addition of a stoichiometric amount of water ( Figure  2b). 3,5,6,16,18,19,21,41−44 In the hydrolytic transformation, the high Brønsted basicity of alkylzinc moieties is not only used to generate O 2− ions but also facilitates initial deprotonation of the proligand. This approach enabled the synthesis of μ 4 -oxido complexes incorporating a wide range of organic ligands, namely, carboxylates, 5,16,21 phosphinates, 18,19,42 amidates, 23 amidinates, 6,26,41,43 and guanidinates. 44 Despite the wide scope of applied [Zn 4 (μ 4 -O)] 6+ core coating ligands, this relatively universal approach for Zn−oxido complexes is essentially non-transferable to the related transition-metal systems.   In contrast to zinc−oxido complexes, tetrahedral open-shell transition metal−oxido complexes were mostly obtained serendipitously, seldom by design. 45 For example, Mn(II), Fe(II), and Co(II) μ 4 -oxido clusters stabilized by N,N′bidentate ligands were isolated from reactions of inorganic metal salts with lithiated ligands. 46−48 In all cases, contamination by atmospheric oxygen or moisture was indicated as the source of the O 2− ions. Similarly, a 3,5-dimethylpyrazolate cobalt-oxido cluster was obtained unexpectedly during a t t e m p t s t o s y n t h e s i z e t h e mi x e d c o b a l t / z i n c [Co 1/3 Zn 2/3 (Hdmpz) 2 ] x (Hdmpz = 3,5-dimethylpyrazole) complex. 49 Furthermore, to the best of our knowledge, the carbamate cluster [Co 4 (μ 4 -O) (OOCNC 9 H 18 ) 6 ], obtained via the insertion of a TEMPO radical into Co 2 (CO) 8 7,50 is the only [M 4 (μ 4 -O)L 6 ] system characterized magnetically. Therefore, we believe that the application potential of tetrahedral oxido-centered clusters as functional materials cannot be fully revealed without a reliable synthesis method applicable to metal centers of various characters.
Herein, we present a novel one-pot transmetalation/ hydrolysis procedure for the synthesis of homometallic oxido clusters incorporating various divalent metal centers ( Figure  2c). The efficiency of the developed approach was demonstrated by the preparation of an amide-stabilized zinc-oxido complex [Zn 4 (μ 4 -O)(NHCOPh) 6 ] (1-Zn) and its hitherto inaccessible iron (II) and cobalt (II) analogues (1-Fe and 1-Co). The structural characterization of 1-Fe, 1-Co, and 1-Zn revealed that they all exhibit similar self-assembly properties resulting in a honeycomb-like supramolecular motif. Furthermore, magnetic characterization of 1-Fe and 1-Co shows strong antiferromagnetic interactions between the metal centers within the [M 4 (μ 4 -O)] core likely mediated by the tetrahedral O 2− bridges.
■ EXPERIMENTAL SECTION General Considerations. All manipulations were conducted under a dry, oxygen-free argon atmosphere either using standard Schlenk techniques or in a glovebox. All reagents were purchased from commercial vendors: benzamide (Sigma), KH (Sigma), cobalt (II) chloride (ABCR), iron (II) chloride (ABCR), zinc chloride and used as received. Solvents were purified using an MBraun SPS-5 system.
Characterization. FTIR spectra were measured with a Bruker Tensor II spectrometer using the ATR technique. Elemental analyses were performed using a UNICUBE elemental analyzer (Elementar Analysensysteme GmbH). Powder XRD data were collected on a PANalytical Empyrean diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5406 Å) or Bruker D8 Advance diffractometer with V-filtered Cr Kα radiation (λ = 2.2897 Å). The sample was spread on a surface of a silicon plate fixed to the sample holder and sealed by a Capton tape. Diffraction patterns were collected by scanning with a step of 0.02°. X-ray Structure Determination. The data were collected at 100(2) K on a Nonius Kappa CCD diffractometer 51 using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The crystal was mounted in a nylon loop in a drop of silicon oil to prevent the possibility of decay of the crystal during data collection. The unit cell parameters were determined from ten frames and then refined on all data. The data were processed with DENZO and SCALEPACK (HKL2000 package). 52 The structure was solved by direct methods using the SHELXS-97 program and was refined by full matrix leastsquares on F 2 using the program SHELXL-97. 53 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were introduced at geometrically idealized coordinates with a fixed isotropic displacement parameter equal to 1.5 (methyl groups) times the value of the equivalent isotropic displacement parameter of the parent carbon. Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as a supplementary publication. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ ccdc.cam.ac.uk). CCDC: 2122831 (1-Fe); 2122830 (1-Co); 2122832 (1-Zn).
Magnetic Measurements. Magnetic properties were determined using a Quantum Design MPMS−5XL SQUID magnetometer for direct current (dc) and alternating current (ac) measurements. Microcrystalline samples of 1-Fe and 1-Co were compacted and immobilized into cylindrical PTFE sample holders. Experimental dc data were recorded at 0.1 T in the temperature range 2.0−290 K and at 2.0 K in the field range 0.1−5.0 T. Experimental ac data were collected at a zero static bias field in the temperature range 2.0−50 K and frequency range 3−1000 Hz using an amplitude of B ac = 3 G. However, no relevant out-of-phase signals were detected for either compound. All data were corrected for the diamagnetic contributions of the sample holders and the compound (χ m,dia /10 −4 cm 3 mol −1 = In a typical procedure, equimolar amounts of KH and benzamide were placed in a Schlenk flask and dispersed in THF. The reaction mixture was stirred overnight, and then, a 0.5 molar equivalent of MCl 2 (M = Fe, Co, Zn) was added. After another 12 h, the obtained red-brown (Fe), deep blue (Co), or white (Zn) suspensions were hydrolyzed using the 1:4 H 2 O/M molar ratio. To increase the solubility of the resulting metal−oxido complexes, DMF was added to the reaction mixture, and the solution was stirred overnight. Then, the KCl slurry was removed by filtration, and the respective product was isolated as well-shaped red-brown (Fe), deep blue (Co), or colorless (Zn) crystals ( Figure S8) by slow diffusion of hexane vapor into the parent solution (for systematic characterization of the resulting complexes vide infra).
A key aspect of the developed process is the highly efficient metathesis transmetalation driven by the low solubility of KCl in the employed organic solvent. The use of a potassium salt also allows to effectively eliminate the incorporation of alkali ions into the final product, which was reported to occur in the case of Li-or Zn-based salts used in transmetalation processes. 22,39,43,54−56 Advantageously, potassium salts of highly basic ligands like amides can be easily generated in situ by the reaction of a selected proligand L-H with KH, where the only side product is H 2 , which finally enables a onepot synthesis. Furthermore, the utilization of ligands with sufficiently high basicity is a crucial factor for ensuring proper reactivity of a homoleptic ML 2 complex, which acts as both (i) the source of M(II) ions and stabilizing ligands, as well as (ii) the water-deprotonating agent. Overall, the transmetalation/ hydrolysis process leads to the desired product [M 4 (μ 4 -O)L 6 ] in high purity, with the generation of three easy-to-separate side products: (i) gaseous H 2 leaving the reaction system, (ii) KCl precipitate that can be removed by filtration, and (iii) the starting proligand, which can be separated from the product by crystallization and reused. Due to the above-mentioned qualities, the developed methodology is potentially easily applicable to other μ 4 -O systems based on various M(II) metal centers and a wide range of organic ligands with high basicity such as amidinate, ureate, or guanidinate anions.
Structural Characterization. Complexes 1-Fe, 1-Co, and 1-Zn were characterized spectroscopically and by single-crystal X-ray diffraction and elemental analysis. Compound 1-Zn was previously obtained via the organometallic synthesis, 23 Figure S5). Furthermore, each of these isomers is chiral, which adds up to four pairs of possible enantiomers occupying the same position in the crystal lattice. This coordination position isomerism introduces differences in environments around N-bonded protons, which explains the observed respective signal splitting in the 1 H NMR spectrum ( Figure S10, for details see Supporting Information).
Very recently, we have demonstrated that the N-bonded hydrogen atoms in the proximal secondary coordination sphere provide efficient H-donor sites for hydrogen bonds which influence the self-assembly of zinc-oxido clusters. 6 In this context, we showed that complex 1-Zn, comprising both O and NH groups in the anchoring group of stabilizing ligand, selfassembles into 2D supramolecular honeycomb-like layers via pairs of complementary intermolecular NH···O hydrogen bonds (the N−H···O distance is 2.760 Å) supported by π−π interactions in the distal secondary coordination sphere (Figures 4 and S4). The new complexes 1-Fe and 1-Co exhibit similar self-assembly properties with the corresponding intermolecular O···HN distances of 2.662 and 2.783 Å, respectively (Table S10), as well as analogous packing of Magnetic Characterization. Comprehensive characterization of physicochemical properties of molecular building blocks is crucial for designing new functional supramolecular assemblies. Notably, the exploration of regular [M 4 (μ 4 -O)L 6 ]type magnetic systems with uniform exchange pathways (i.e., coupling patterns) between all spin centers has been impeded by the inaccessibility of well-defined model complexes. To the best of our knowledge, the carbamate cluster [Co 4 (μ 4 -O)(OOCNC 9 H 18 ) 6 ] is the only system of this type characterized magnetically, albeit on a purely phenomenological level indicating antiferromagnetic interaction between the cobalt ions. 7 However, tetranuclear Cu(II) cluster [Cu 4 (μ 4 -O)(OOCCF 3 ) 6 (q) 4 ] (q = quinoline) can be described as a farther-related analogue of such regular systems, based on pentacoordinate, trigonal bipyramidal metal centers. 58 Its magnetic properties were extensively studied experimentally and theoretically indicating exceptionally ferromagnetic interactions within the [M 4 (μ 4 -O)] core. All other works on the magnetic properties of μ 4 -oxido-centered complexes involve more distorted systems with less uniform ligand spheres, commonly including monoatomic chalcogenide or alkoxide bridges prone to the mediation of strong antiferromagnetic interactions. 59   Inorganic Chemistry pubs.acs.org/IC Article reasonable approximation since tetrahedrally coordinated 3d 6 and 3d 7 centers are characterized by 5 E and 4 A 2 ground terms, respectively. In contrast, due to the larger spin−orbit coupling of both centers, the ground terms are usually split, leading to temperature-dependent behavior of χ m T below 30 K. However, this feature can be neglected for 1-Co as a relevant splitting of the A 2 ground term requires a strong distortion from the tetrahedral ligand field, which is not the case for the molecular structure of 1-Co. For 1-Fe; however, the quintet E term is significantly split in any case. Therefore, the χ m T versus T data for both compounds were simultaneously fitted at both 0.1 and 1.0 T, restricted to the temperature interval of 10−290 K and subsequently the values at lower temperatures were calculated. Based on the single-crystal X-ray structures, we model the M 4 cluster as a trigonal pyramid, with the apex defined by threefold N-coordinated metal, since the distance from the central O here slightly differs from the other centers (as shown in Figure 3). Accordingly, we define the spin connectivity by two different exchange parameters: J b , between the metal centers of the base (denoted by indices 2−4) and J t , between a metal center of the base and the apical (top) metal center (index 1). Thus, the corresponding effective Hamiltonian is The data of 1-Fe indicate a small level of paramagnetic impurities, which was accounted for in the analysis of the data. For 1-Co, we neglected such contributions, since the measurements point to a negligible level, that is, within the error margin of the fitting model. For 1-Fe, consisting of effective S = 2 centers, the least-squares fit yields J t = (−18.6 ± 0.1) cm −1 , J b = (−15.7 ± 0.2) cm −1 , and g eff = 2.21 ± 0.02, and a paramagnetic impurity level corresponding to 0.0964 Fe II centers per formula unit. For 1-Co (effective S = 3/2 centers), we find J t = (−24.5 ± 0.1) cm −1 , J b = (−19.6 ± 0.1) cm −1 , and g eff = 2.56 ± 0.02. In both cases, the exchange interactions are antiferromagnetic, and the ground state of the respective compound can be characterized by an effective total spin S total of 0. For each 1-Fe and 1-Co, the obtained two J values correspond to the geometric differences, in particular the M− (μ 4 -O)−M angles. Furthermore, the J values for the Fe II and Co II systems are very comparable when considering the different spin quantum numbers that enter the effective spin Hamiltonian as S 2 . The effective g factors are fully in line with tetrahedrally coordinated Fe II and Co II centers.

■ CONCLUSIONS
We have developed a new efficient one-pot synthetic approach for tetrahedral μ 4 −oxido complexes of various divalent metal centers stabilized by monoanionic bridging O,N-ligands. The new synthetic procedure relies on simple stoichiometric hydrolysis of the homoleptic [ML 2 ] complex generated in situ in a transmetalation reaction. To prove the usefulness of the developed approach, we have synthesized and characterized a series of M(II) (M = Fe, Co, Zn) oxido benzamidates. All complexes exhibit analogous honeycomblike supramolecular structures in their crystal lattices, which indicates that their self-assembly properties are dominated by cooperative noncovalent interactions in the secondary coordination sphere and likely unaffected by the specific character of the metal center. Moreover, we have characterized the magnetic properties of the new Fe(II) and Co(II) complexes, utilizing effective spin models that underscore a general tendency for antiferromagnetic coupling of μ 4 -bridged metal centers in [M 4 (μ 4 -O)]-type systems.
The reported procedure is potentially easily extensible to other M(II)−oxido systems stabilized by a vast array of organic ligands. These findings will likely pave the way to new oxido clusters with interesting optoelectronic and novel molecular building units and, as a result, will allow for the development of new, as of yet elusive functional materials.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c00456. SC-XRD data, additional description of molecular structures and magnetic properties, PXRD patterns, and FTIR and NMR spectra (PDF)